1. Field of the Invention
The present invention relates to automatic gain control (AGC) and in particular to techniques used in digital AGC that can quickly sense the signal size over a large dynamic range and avoid digital filter settling time after gain change.
2. Description of the Related Art
In a typical wireless communication system, a receiver has one or multiple variable gain stages before ADC. For example, FIG. 1 illustrates a simplified receiver 100 for receiving signals in a WLAN environment. In receiver 100, a bandpass filter 102 receives the incoming signals from an antenna 101 and outputs a predetermined band of frequencies (while excluding those frequencies higher and lower than the predetermined band). A variable gain RF amplifier 103 can provide an initial amplification to that predetermined band of frequencies. A mixer 104 converts those amplified signals (using a signal from a local oscillator) into intermediate frequency (IF) signals, which are then amplified by an IF amplifier 105.
In one embodiment, mixers 106 can include an in-phase mixer directly driven by a local oscillator and a quadrature mixer driven by the same local oscillator signal after it is phase-shifted by 90° in a phase shifter. In this way, in-phase (I) and quadrature (Q) components of the amplified IF signal are obtained at the outputs (only one shown) of mixers 106.
At this point, low-pass filters 107 (including both I and Q branches) can generate signals in the desired channel (called the baseband signals). Amplifiers 108 then amplify these baseband (BB) signals. Analog to digital converters (ADCs) 110 (provided for both the I and Q branches of low-pass filters 107) transform the amplified baseband signals into digital signals that can be analyzed by a processing block 111. ADCs 110 can be implemented as pipeline ADCs, sigma-delta converters, or any other mechanisms for converting analog signals to digital signals.
Amplifiers 103, 105, and 108 can advantageously amplify the received signal such that a weak signal can be distinguished from noise (i.e. provide a high signal to noise ratio (SNR)). However, too much amplification can adversely affect receiver 100. For example, an amplified strong signal can distort incoming signals, thereby overloading and possibly damaging certain components (e.g. saturating ADCs 110). For this reason, automatic gain control (AGC) 112 tries to maintain the amplified signals within certain ranges. Specifically, AGC 112 can use the magnitude of the digital signal, as detected by ADCs 110, to adjust the gains of amplifiers 103, 105, and 108. In a typical embodiment, AGC 112 uses the same gain adjustment value for RF amplifier 103, IF amplifier 105, and BB amplifiers 108.
Typically, a system initially uses a high gain when no signals are detected, thereby ensuring that any transmitted signals can be easily detected. However, if a large signal is then received, the ADCs can be saturated. For example, FIG. 2 illustrates an exemplary signal range 200 from −20 dB to −110 dB (i.e. a 90 dB range). If the gain of the amplifiers is set so as to permit detection of a signal at −110 dB, then a large signal could easily saturate the ADC. When the ADCs saturate, a conventional correction technique can consecutively drop the largest signal size by a first predetermined amount (e.g. 10 dB) until the ADCs do not saturate. FIG. 2 shows that three drops 201, 202, and 203 have been made.
This trial and error technique wastes significant time. Specifically, the gain of the amplifiers is ideally determined during the preamble of the packet, thereby allowing immediate decoding of the data (which follows the preamble). Unfortunately, this gain control technique can take longer than receipt of the preamble. In this case, the received data may be corrupted due to either ADC saturation or insufficient SNR.
Notably, AGC 112 can include narrow digital filters to filter out out-of-band interferences (called blockers) and noise. Narrow digital filters have a long settling time. Unfortunately, in, for example, a personal handy phone system (PHS), for each gain change made by AGC 112, digital signal processing must be delayed until these filters settle. Thus, yet more time is wasted due to digital filter settling.
Therefore, a need arises for a gain control technique that can quickly sense the signal size over a large dynamic range and minimize the digital filter settling time after a gain change. This improved gain control technique would be finished within the preamble and would also allow time for other tasks such as synchronization and impairment estimation.